Synthetic peptides having pituitary growth hormone releasing activity

ABSTRACT

Novel pentapeptides having the following amino acid sequence X-Y-Z-E-G-J-L wherein X is selected from a group consisting of -NH2, -NHCH3, and -N(CH3)2; wherein Y and G are independently selected from a group consisting of tyrosine, tryptophan, and phenylalanine; Z is selected from a group consisting of glycine, alanine, valine, leucine, isoleucine, proline, hydroxyproline, serine, threonine, cysteine, and methionine; E and J are independently selected from a group consisting of D-tyrosine, D-tryptophan, and D-phenylalanine; and L is a C-terminal functional group selected from a group consisting of amide, amide lower alkyl, amide di(lower alkyl), lower alkoxy, hydroxy, and carboxy and the lower ester derivatives thereof; and the pharmaceutically acceptable salts thereof. These peptides act directly on the pituitary to release growth hormone therefrom.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pentapeptides which possess pituitary growthhormone releasing activity.

2. Description of the Prior Art

Growth hormone, which is secreted from the pituitary, causes growth ofall tissues of the body that are capable of growing. In addition, growthhormone is known to have the following basic effects on the metabolicprocess of the body:

1. Increased rate of protein synthesis in all cells of the body;

2. Decreased rate of carbohydrate utilization in cells of the body;

3. Increased mobilization of free fatty acids and use of fatty acids forenergy.

A deficiency in growth hormone secretion can result in various medicaldisorders, such as some instances of dwarfism.

Various ways are known to release growth hormone. For example, chemicalssuch as argine, L-dihydroxyphenylamine (L-DOPA), glucagon, vasopressin,and insulin induced hypoglycemia, as well as activities such as sleepand exercise, indirectly cause growth hormone to be released from thepituitary by acting in some fashion on the hypothalmus perhaps either todecrease somatostatin secretion or to increase an unknown endogenousgrowth hormone-releasing hormone or both.

Compounds which directly act on the pituitary to release growth hormoneinclude prostaglandin E₁ and E₂, theophylline, and cyclic nucleotides.However, these compounds neither specifically release growth hormone norare they believed to act at the putative growth hormone-releasinghormone receptors in the peripheral membrane of the pituitary cell toinitiate growth hormone release.

In addition, under special conditions certain chemically definedpeptides, e.g., vasopressin, thyroid-releasing hormone (TRH),luteinizing hormone-releasing hormone (LH-RH), α-melanocyte-stimulatinghormone (α-MSH), glucagon, substance P, neurotensin; Met-enkephelin,β-endorphin, cholera-enderotoxin, and basic myelin protein, act torelease growth hormone from the pituitary. However, only TRH actsdirectly on the pituitary to elicit this response. Furthermore, theabove listed peptides release other pituitary hormones and under mostexperimental conditions do not release growth hormone. For example, TRHdoes not release growth hormone in normal rats or in normal humans orfrom pituitaries of normal rats or monkeys. In vitro, TRH releasesgrowth hormone, prolactin, and thyroid stimulating hormone (TSH) and invivo TRH releases these hormones from bovine pituitary.

Vasopressin's induced release of growth hormone is considered to be dueto a non-specific response to stress caused by administration of highdosages of vasopressin.

Accordingly it would be highly desirable to have a compound whichdirectly acts on the pituitary under normal experimental conditions toeffect the release of growth hormone therefrom. Such peptides would beuseful in vitro as unique research tools for understanding how growthhormone secretion is regulated at the pituitary level and would also beuseful in vivo to treat symptoms related to growth hormone deficiencies.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided peptideswhich act directly on the pituitary under normal experimental conditionsin vitro to release growth hormone therefrom.

These growth hormone releasing pentapeptides can be utilized in vitro asunique research tools for understanding, inter alia, how growth hormonesecretion is regulated at the pituitary level.

Also, the growth hormone releasing pentapeptides of the instantinvention can also be administered in vivo to increase growth hormonerelease.

More particularly, this invention encompasses novel peptides having theformula

    X-Y-Z-E-G-J-L

wherein X is selected from a group consisting of --NH₂, --NHCH₃, and--N(CH₃)₂ ; wherein Y and G are independently selected from a groupconsisting of tyrosine, tryptophan and phenylalanine; Z is selected froma group consisting of glycine, alanine, valine, leucine, isoleucine,proline, hydroxyproline, serine, threonine, cysteine, and methionine; Eand J are independently selected from a group consisting of D-tyrosine,D-tryptophan, and D-phenylalanine; and L is a C-terminal functionalgroup selected from a group consisting of an amide, amide lower alkyl,amide di(lower alkyl), lower alkoxy, hydroxy, and carboxy and the lowerester derivatives thereof; and the pharmaceutically acceptable saltsthereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pentapeptides of this invention have the amino acid sequencerepresented by formula I:

    X-Y-Z-E-G-J-L                                              (I)

wherein X is selected from a group consisting of --NH₂, --NHCH₃, and--N(CH₃)₂ ; wherein Y and G are independently selected from a groupconsisting of tyrosine, tryptophan, and phenylalanine; Z is selectedfrom a group consisting of glycine, alanine, valine, leucine,isoleucine, proline, hydroxyproline, serine, threonine, cysteine, andmethionine; E and J are independently selected from a group consistingof D-tyrosine, D-tryptophan, and D-phenylalanine; and L is a C-terminalfunctional group selected from a group consisting of amide (--CONH₂),amide lower alkyl (--NHR), amide di(lower alkyl) (--CONR₁ R₂), loweralkoxy (--CH₂ OR), hydroxy (--CH₂ OH), carboxy (--COOH) and the lowerester derivatives thereof (--COOR) and the pharmaceutically acceptablesalts thereof.

Preferably, the pentapeptides of this invention have the amino acidseuence represented by formula II:

    X-Tyr-Gly-D-Trp-Phe-D-Phe-L                                (II)

wherein X and L are as defined above.

All amino acid residues identified herein are in the natural orL-configuration unless otherwise specified.

Abbreviations for amino acid residue have been used in accordance withthe following standard peptide nomenclature:

    ______________________________________                                        Tyr                                                                           L-tyrosine           Ile                                                      L-isoleucine                                                                  D--Tyr                                                                        D-tyrosine                                                                           D--Ile                                                                 D-isoleucine                                                                  Gly                                                                           glycine                                                                              Leu                                                                    L-leucine                                                                     Phe                                                                           L-phenylalanine                                                                      D--Leu                                                                 D-leucine                                                                     D--Phe                                                                        D-phenylalanine                                                                      Thr                                                                    L-threonine                                                                   Met                                                                           L-methionine                                                                         D--Thr                                                                 D-threonine                                                                   D--Met                                                                        D-methionine                                                                         Val                                                                    L-valine                                                                      Ala                                                                           L-alanine                                                                            D--Val                                                                 D-valine                                                                      D--Ala                                                                        D-alanine                                                                            Pro                                                                    L-proline                                                                     Ser                                                                           L-serine                                                                             D--Pro                                                                 D-proline                                                                     D--Ser                                                                        D-serine                                                                             Gln                                                                    L-glutamine                                                                   Lys                                                                           L-lysine                                                                             D--Gln                                                                 D-glutamine                                                                   D--Lys                                                                        D-lysine                                                                             Glu                                                                    L-glutamic acid                                                               Asn                                                                           L-asparagine                                                                         D--Glu                                                                 D-glutamic acid                                                               D--Asn                                                                        D-aspargine                                                                          Trp                                                                    L-tryptophan                                                                  His                                                                           L-histidine                                                                          D--Trp                                                                 D-tryptophan                                                                  D--His                                                                        D-histidine                                                                          D--Asp                                                                 D-aspartic acid                                                               Cys                                                                           L-cysteine                                                                           L--Arg                                                                 L-arginine                                                                    D--Cys                                                                        D-cysteine                                                                           D--Arg                                                                 D-arginine                                                                    Hypro                                                                         L-hydroxyproline                                                                     D--Hypro                                                               D-hydroxyproline                                                              ______________________________________                                    

The term "pharmaceutically acceptable salts," as used herein, refers tothe non-toxic alkali metal, alkaline earth metal and ammonium saltscommonly used in the pharmaceutical industry including the sodium,potassium, lithium, calcium, magnesium, barium, ammonium and protaminezinc salts which are prepared by methods well known in the art. The termalso includes non-toxic acid addition salts which are generally preparedby reacting the compounds of this invention with a suitable organic orinorganic acid. Representative salts include the hydrochloride,hydrobromide, sulfate, bisulfate, acetate, oxalate, valerate, oleate,laurate, borate, benzoate, lactate, phosphate, tosylate, citrate,maleate, fumarate, succinate, tartrate, napsylate, and the like.

The term "lower alkyl," as used herein, refers to straight and branchedchain alkyl groups having from 1 to 6 carbon atoms, such as methyl,ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl, sec-butyl, n-pentyl,n-hexyl, 1,2-dimethylbutyl, and the like. Preferably, the lower alkylgroup is methyl or ethyl.

The term "lower alkoxy", as used herein, refers to straight and branchedchain alkoxy groups having from 1 to 6 carbon atoms, such as methoxy,ethoxy, n-propoxy, iso-propoxy, n-butoxy, tert-butoxy, sec-butoxy,n-pentoxy, n-hexoxy, 1,2-dimethylbutoxy and the like. Preferably, thelower alkoxy group is methoxy or ethoxy.

The term "lower ester derivative", as used herein, refers to straightand branched chain alkyl ester derivatives having from 1 to 6 carbonatoms, such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl,sec-butyl, n-pentyl, n-hexyl, and 1,2-dimethylbutyl ester derivativesand the like. Preferably, the lower ester derivative is a methyl esterderivative or an ethyl ester derivative.

Accordingly, each R, R₁, and R₂ is independently selected from a groupconsisting of straight and branched chain alkyl groups containing 1-6carbon atoms. Preferably, each R, R₁, and R₂ are independently selectedfrom the group consisting of alkyl group containing 1-2 carbon atoms.

Table I sets forth peptides within the scope of the instant invention.

                  TABLE I                                                         ______________________________________                                        X--Tyr--Gly--D-Trp-- Phe--D-Phe-- CONH.sub.2                                  X--Tyr--Gly--D-Trp-- Phe--D-Phe-- CONHR                                       X--Tyr--Gly--D-Trp-- Phe--D-Phe-- CONR.sub.1 R.sub.2                          X--Tyr--Gly--D-Trp-- Phe--D-Phe-- CH.sub.2 OR                                 X--Tyr--Gly--D-Trp-- Phe--D-Phe-- CH.sub.2 OH                                 X--Tyr--Gly--D-Trp-- Phe--D-Phe-- COOH                                        X--Tyr--Gly--D-Trp-- Phe--D-Phe--COOR                                         ______________________________________                                    

The peptides of the instant invention can be prepared by classicalmethods known in the art or, preferably, by using standard solid-phasetechniques. The synthesis is commenced from the C-terminal end of thepeptide using an α-amino protected resin. A suitable starting materialcan be prepared, for instance, by attaching the required α-amino acid toa chloromethylated resin, a hydroxymethyl resin, or a benzhydrylamineresin. One such chloromethylated resin is sold under the tradenameBIO-BEADS SX-1 by Bio Rad Laboratories, Richmond, Calif. and thepreparation of the hydroxymethyl resin is described by Bodonszky et al.,Chem. Ind. (London) 38, 1597 (1966). The benzhydrylamine (BHA) resin hasbeen described by Pietta and Marshall, Chem. Commn. 650 (1970) and iscommercially available from Beckman Instruments, Inc., Palo Alto, Calif.in the hydrochloride form thereof (BHA.HCl).

In the preparation of the compounds of this invention, an α-aminoprotected amino acid is coupled to the chloromethylated resin with theaid of, for example, cesium bicarbonate catalyst, according to themethod described by Gisin, Helv. Chim. Acta, 56, 1467 (1973). After theinitial coupling, the α-amino protecting group is removed by a choice ofreagents including trifluoroacetic acid (TFA) or hydrochloric acid (HCl)solutions in organic solvents at room temperature. After removal of theα-amino protecting group, the remaining protected amino acids arecoupled stepwise in the desired order. Each protected amino acid isgenerally reacted in about a 3-fold excess using an appropriate carboxylgroup activator such as dicyclohexylcarbodiimide (DCC) in solution, forexample, in methylene chloride (CH₂ Cl₂)-dimethylformamide (DMF)mixtures.

After the desired amino acid sequence has been completed, the desiredpeptide is decoupled from the resin support by treatment with a reagentsuch as hydrogen fluoride (HF) which not only cleaves the peptide fromthe resin, but also cleaves all remaining side-chain protecting groups.When the chloromethylated resin is used, hydrogen fluoride treatmentresults in the formation of the free peptide acids of Formula I(Y=--COOH). When the benzhydrylamine resin is used, hydrogen fluoridetreatment results directly in the free peptide amides of Formula I(Y=--CONH₂). Alternatively, when the chloromethylated resin is employed,the side-chain protected peptide can be decoupled by treatment of thepeptide-resin with ammonia to give the desired side-chain protectedamide or with an alkylamine to give a side-chain protected alkylamide ordialkylamide. Side-chain protection is then removed in the usual fashionby treatment with hydrogen fluoride to give the free amides,alkylamides, or dialkylamides.

In preparing the esters of this invention the resins used to prepare theacids of Formula I (Y=--COOH) are employed and the side-chain protectedpeptide is cleaved with base and the appropriate alcohol, i.e.,methanol. Side-chain protecting groups are then removed in the usualfashion by treatment with hydrogen fluoride to obtain the desired ester.

The solid-phase procedure discussed above is well known in the art andhas been essentially described by J. M. Stewart, Solid Phase PeptideSynthesis: (Freeman and Co., San Francisco, 1969).

The growth hormone releasing pentapeptides of Formula I are useful invitro as unique tools for understanding how growth hormone secretion isregulated at the pituitary level. This includes use in the evaluation ofmany factors thought or known to influence growth hormone secretion suchas age, sex, nutritional factors, glucose, amino acids, fatty acids, aswell as fasting and non-fasting states. In addition, the pentapeptide ofthis invention can be used in the evaluation of how other hormonesmodify growth hormone releasing activity. For example, it has alreadybeen established that somatostatin inhibits growth hormone release.Other hormones that are important and in need of study as to theireffect on growth hormone release include the gonadal hormonestestosterone, estradiol, and progesterone; the adrenal hormones cortisoland other corticoids, epinephrin and norepinephrine; the pancreatic andgastrointestine hormones, insulin, glucagon, gastric, secretion, thevasoactive intestinal peptides, i.e., bombesin; and the thyroid hormonesthyroxine and triiodothyronine. The pentapeptides of Formula I can alsobe employed to investigate the possible negative or positive feedbackeffects of some of the pituitary hormones, e.g., growth hormone andendorphin peptides, on the pituitary to modify growth hormone release.Of particular scientific importance is the use of these pentapeptides toelucidate the subcellular mechanisms mediating the release of growthhormone.

The pentapeptides of Formula I can also be administered to warm bloodedanimals, including man, to release growth hormone in vivo. For example,the pentapeptides of Formula I can be administered to treat symptomsrelated to growth hormone deficiencies. In addition, these pentapeptidescan be administered to commercially important animals to accelerate andincrease their rate and extent of growth.

Accordingly, the present invention includes within its scopepharmaceutical compositions comprising, as an active ingredient, atleast one of the compounds of Formula I in association with apharmaceutical carrier or diluent. The compounds of this invention canbe administered by oral, parenteral (intramuscular, intraperitoneal,intravenous (IV) or subcutaneous injection), nasal, vaginal, rectal orsublingual routes of administration and can be formulated in dosageforms appropriate for each route of administration.

Solid dosage forms for oral administration include capsules, tablets,pills, powders and granules. In such solid dosage forms, the activecompound is admixed with at least one inert pharmaceutically acceptablecarrier such as sucrose, lactose, or starch. Such dosage forms can alsocomprise, as is normal practice, additional substances other than inertdiluents, e.g., lubricating agents such as magnesium stearate. In thecase of capsules, tablets and pills, the dosage forms may also comprisebuffering agents. Tablets and pills can additionally be prepared withenteric coatings.

Liquid dosage forms for oral administration include pharmaceuticallyacceptable emulsions, solutions, suspensions, syrups, the elixirscontaining inert diluents commonly used in the art, such as water.Besides, such inert diluents, compositions can also include adjuvants,such as wetting agents, emulsifying and suspending agents, andsweetening, flavoring, and perfuming agents.

Preparations according to this invention for parental administrationinclude sterile aqueous or non-aqueous solutions, suspensions, oremulsions. Examples of non-aqueous solvents or vehicles are propyleneglycol, polyethylene glycol, vegetable oils, such as olive oil and cornoil, gelatin, and injectable organic esters such as ethyl oleate. Suchdosage forms may also contain adjuvants such as preserving, wetting,emulsifying, and dispersing agents. They may be sterilized by, forexample, filtration through a bacteria-retaining filter, byincorporating sterilizing agents into the compositions, by irradiatingthe compositions, or by heating the compositions. They can also bemanufactured in the form of sterile water, or some other sterileinjectable medium immediately before use.

Compositions for rectal or vaginal administration are preferablysuppositories which may contain, in addition to the active substance,excipients such as cocoa butter or a suppository wax.

Compositions for nasal or sublingual administration are also preparedwith standard excipients well known in the art.

The dosage of active ingredient in the compositions of this inventionmay be varied; however, it is necessary that the amount of the activeingredient shall be such that a suitable dosage form is obtained. Theselected dosage depends upon the desired therapeutic effect, on theroute of administration, and on the duration of the treatment.Generally, dosage levels of between 0.001 to 10 mg/kg. of body weightdaily are administered to mammals to obtain effective release of growthhormone.

The following examples are provided for the purpose of furtherillustration only and are not intended to be limitations on thedisclosed invention.

EXAMPLE 1 Synthesis of H₂ N-Tyr-Gly-D-Trp-Phe-D-Phe-CONH₂

BHA.HCl resin was placed in a reaction vessel. The following procedurewas then employed in conjunction with a Beckman brand PeptideSynthesizer Model No. 990 in preparing the pentapeptide H₂N-Tyr-Gly-D-Trp-Phe-D-Phe-CONH₂ :

1. Methylene chloride (CH₂ Cl₂ ; about 10 ml/gm of BHA.HCl resin) wasadded to the reaction vessel. The BHA.HCl resin was washed with vigorousstirring for about 1.5 minutes. The CH₂ Cl₂ solution was then drainedfrom the reaction vessel. This washing step was repeated once.

2. A triethylamine solution ((Et₃ N)/CH₂ Cl₂ (10:90); about 10 ml/gmBHA.HCl resin) was added to the washed BHA.HCl resin in the reactionvessel. The resulting mixture was vigorously stirred for about 1.5minutes. The solution was then drained from the reaction vessel.

3. Another Et₃ N₃ /CH₂ Cl₂ (10:90) solution (about 10 ml/gm BHA.HCl) wasadded to the reaction vessel. The BHA.HCl resin resin was neutralized byvigorous stirring for about 20 minutes. The solution was then drainedfrom the reaction vessel.

4. CH₂ Cl₂ (about 10 ml/gm of BHA.HCl resin) was added to the reactionvessel. The resulting mixture was vigorously stirred for about 1.5minutes. The solution was then drained from the reaction vessel. Thisprocedure was repeated an additional two times.

5. An 0.5 molar (M) dicyclohexylcarbodiimide (DCC) in CH₂ Cl₂ solution(about 2.5 times the theoretical amount of total binding capacity of theBHA.HCl resin originally placed in the reaction vessel) was added to thereaction vessel. The resulting mixture was vigorously stirred for about1.5 minutes. The solution was then drained from the reaction vessel.

6. Tertiarybutyloxycarbonyl-D-phenylalanine (Boc-D-Phe; about 2.5 timesthe theoretical amount of the total binding capacity of the BHA.HClresin originally placed in the reaction vessel) in about 50 ml of aDMF-CH₂ Cl₂ (5:45) solution was added to the reaction vessel. Theresulting mixture was vigorously stirred until a negative ninhydrin testwas obtained (about 120 minutes). The solution was then drained from thereaction vessel.

7. CH₂ Cl₂ (about 10 ml/gm of BHA.HCl resin) was added to the reactionvessel. The resulting solution was vigorously stirred for about 1.5minutes. The solution was then drained from the reaction vessel. Thiswashing procedure was repeated once.

8. DMF (about 10 ml/gm of BHA.HCl resin) was added to the reactionvessel. The resulting mixture was stirred for about 1.5 minutes. Thesolution was then drained from the reaction vessel.

9. CH₂ Cl₂ (about 10 ml/gm of BHA.HCl resin) was added to the reactionvessel. The resulting mixture was vigorously stirred for about 1.5minutes. The solution was then drained from the reaction vessel. Thiswashing procedure was repeated an additional two times.

10. A trifluoroacetic acid (TFA)/CH₂ Cl₂ (40:60) solution (about 10ml/gm of BHA.HCl resin) was added to the reaction vessel. The resultingmixture was vigorously stirred for about 1.5 minutes. The solution wasthen drained from the reaction vessel.

11. Another TFA/CH₂ Cl₂ (40:60) solution (about 10 ml/gm of BHA.HClresin) was added to the reaction vessel. The resulting mixture wasvigorously stirred for about 20 minutes. The solution was then drainedfrom the reaction vessel.

12. Chloroform (CHCl₃ ; about 10 ml/gm of BHA.HCl resin) was added tothe reaction vessel. The resulting mixture was vigorously stirred forabout 1.5 minutes. The solution was then drained from the reactionvessel.

13. An ethanol (EtOH)/CH₂ Cl₂ (30:70) solution (about 10 ml/gm ofBHA.HCl resin) was added to the reaction vessel. The resulting mixturewas vigorously stirred for about 1.5 minutes. The solution was thendrained from the reaction vessel. This washing step was repeated once.

Steps 1 through 13 were then repeated employing the following sequenceof amino acids:

Boc-Phe

Boc-D-Trp

Boc-Gly

Boc-Tyr(BrZ*)

After completion of the synthesis of the desired peptide resin, thereaction vessel containing the peptide resin was then placed in adessicator and dried overnight under a vacuum. The dried peptide resinwas removed from the reaction vessel and placed in another vesselsuitable for HF cleavage. This latter vessel also contained a magneticstirring bar. A quantity of anisol sufficient to wet the peptide resinwas added to this vessel. The vessel was next connected to an HF lineand placed under a vacuum to remove any air therein. The vessel was thencooled to about -78° C. with a dry ice-acetone bath. Doubly distilled HF(about 10 ml/gm of peptide resin) was added to the vessel. The dryice-acetone bath was then removed from the vessel and replaced by anice-water bath. The vessel's contents were vigorously stirred for about45 minutes while the vessel remained immersed in the ice-water bath.Most of the HF in the vessel was then removed by water aspiration. Afterthe majority of HF was removed by water aspiration, the remaining HF andanisole were removed via a vacuum pump.

The vessel's contents were washed with about 100 ml of ether to furtherremove any residual anisole.

The peptide was removed from the resin by extraction with 30% aqueousacetic acid (aq.HOAc). The aq.HOAc was lypholized off to yield a fluffypeptide powder.

The peptide was then purified by partition chromatography employing abutonol: HOAc: water (4:1:5) system.

EXAMPLE 2 In Vitro Growth Hormone Release Study

Female rats of the CD-1 strain were housed in a constant temperatureroom at 24° C. with 14 hours light and 10 hours darkness. The rats werefed Purina brand rat chow ab libitum.

All studies were started between 0800 and 1000 hours.

Pituitaries were removed from 20 day old female rats. Two pituitarieswere incubated at 36° C. in 1 ml of lactated Ringer's solution in 10 mlpolytetrafluoroethylene beakers in a Dubnoff Shaker (90 cycles/min.).The pituitaries were incubated a total of 4 hours (P₁, P₂, I₃, and I₄).After two successive one hour preincubation periods (P₁, P₂), thepeptide was added to the incubation medium hourly for 2 hours (I₃, I₄).

This in vitro study was performed in triplicate and a standardradioimmunoassay of growth hormone present in the incubation medium wasperformed in duplicate. The hormonal activity was calculated as thedifference (Δ) of growth hormone (GH) levels between I₃, I₄ and P₂,i.e., (I₃ -P₂) and (I₄ -P₂). P values were calculated by Students' ttest using six growth hormone values for each value recorded.

The growth hormone radioimmunoassay reagents were distributed by theNational Institute of Arthritis and Metabolite Disease Division (NIAMDD)program. The growth hormone values were calculated in terms of nanograms(ng) of a rat standard with a growth hormone potency of 0.61 units/mg.The results obtained are set forth in Table II.

                                      TABLE II                                    __________________________________________________________________________    IN VITRO GROWTH HORMONE RELEASE.sup.1,2                                       Peptide                     Control                                                                              1μg                                                                              10μg                                                                             100μg                       __________________________________________________________________________    H.sub.2 N--Tyr--Gly--D-Trp-- Phe--D-Phe-- CONH.sub.2                                                      -198± 85                                                                          200 ± 167.sup.3                                                                  1384 ± 120                                                                       1196 ± 194                  __________________________________________________________________________     .sup.1 Given in terms of ngΔGH/ml incubation medium ± standard       error of the mean (SEM)                                                       .sup.2 P Value <0.001 unless otherwise noted                                  .sup.3 P Value not significant                                           

The results set forth in Table II demonstrate that peptides within thescope of the instant invention can induce a significant in vitro releaseof growth hormone from the pituitary.

By introducing various other hormones, e.g., somatostatin, testosterone,cortisol, insulin, etc., into the incubation medium of Example 2, onecan study what effect these latter hormones have on the regulation ofgrowth hormone secretion.

EXAMPLE 3 In Vivo Diagnostic Application

A peptide within the scope of this invention is injected IV into amammal, including a human. Blood samples are taken before and at +15minute intervals after the IV injection for about 1 to about 2 hours.Serum growth hormone levels are measured on each of the blood samples.The rise in growth hormone level is an index of the response. The degreeof the growth hormone response is indicative of whether thehypothalmic-pituitary unit is functioning normally to secrete growthhormone.

This test can be employed for evaluating whether thehypothalamic-pituitary system is normal under a large number ofdifferent clinical and experimental conditions in both healthy anddisease states. The test has application at all ages and in both sexes.

Based on this disclosure, many other modifications and ramificationswill naturally suggest themselves to those skilled in the art. These areintended to be comprehended as within the scope of this invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A peptide having aformula

    X-Tyr-Gly-D-Trp-Phe-D-Phe-L

and the pharmaceutically acceptable non-toxic acid addition saltsthereof, wherein X is selected from a group consisting of --NH₂,--NHCH₃, and --N(CH₃)₂ ; and L is selected from a group consisting of--CONH₂, --CONHR, --CONR₁ R₂, --CH₂ OR, --CH₂ OH, --COOH, and --COOR,wherein each R, R₁, and R₂ is independently selected from a groupconsisting of straight and branch alkyl groups containing 1-6 carbonatoms.
 2. The peptide of claim 1 wherein L is --CONH₂.
 3. The peptide ofclaim 1 wherein L is --CONHR.
 4. The peptide of claim 1 wherein L is--CONR₁ R₂.
 5. The peptide of claim 1 wherein L is --CH₂ OR.
 6. Thepeptide of claim 1 wherein L is --CH₂ OH.
 7. The peptide of claim 1wherein L is --COOH.
 8. The peptide of claim 1 wherein L is --COOR. 9.The peptide of claim 1 of the formula

    H.sub.2 N-Tyr-Gly-D-Trp-Phe-D-Phe-CONH.sub.2.


10. The peptide of claim 1 wherein each R, R₁, and R₂ is independentlyselected from the group consisting of alkyl groups containing 1-2 carbonatoms.
 11. A method of releasing growth hormone from a pituitarycomprising contacting said pituitary with the peptide of claims 2 or 1.12. A compound of the formula

    Boc-Tyr(BrZ)-Gly-D-Trp-Phe-D-Phe-®

wherein BrZ is p-bromobenzyloxycarbonyl; p1 Boc is t-butyloxycarbonyl;and ® is benzhydrylamine resin.